KABUL, Aug 5 (Reuters) - Taliban militants have switched strategy from targeting rural areas of Afghanistan to attacking provincial cities, in response to increased U.S. air strikes after the United States said it was ending its longest war, three militant commanders said.

The Taliban have stepped up their campaign to defeat the U.S.-backed government as foreign forces complete their withdrawal after 20 years of conflict.

A regional U.S. commander said late last month the United States had increased air strikes to counter growing Taliban attacks, a move condemned by the Islamist group.

Fighting has been particularly heavy inside the city of Herat, near the western border with Iran, Lashkar Gah, capital of Helmand province in the southwest, and Kandahar in the south.

The three Taliban commanders, who spoke on condition of anonymity, told Reuters that they were focused on capturing Herat and Kandahar, with Lashkar Gah in their sights.

"Mullah Yaqoob argued that when U.S. didn't fulfil their commitment why should Taliban be made to follow the accord?" said one of the commanders, based in Kandahar, referring to the group's military chief.

"Mullah Yaqoob has decided to capture Kandahar and Herat and now Helmand and then it could be Kunduz, Khost or any other province," said the commander, saying the military leader’s arguments had won over the group's political office.

A Taliban spokesman did not respond to a request for comment. Taliban negotiator Suhail Shaheen told Reuters the group was continuing its policy of seizing control of rural areas and implementing Islamic Sharia there, rather than focusing on cities.

The Taliban, who ruled with an iron hand from 1996 until 2001, had said previously they would focus on lucrative border crossings and large rural areas, though they have encircled and at times entered provincial capitals.

The group has been waging a massive nationwide offensive since April when President Joe Biden announced troops would withdraw by September and as officials warned peace talks in Doha were failing to make substantive progress.

In recent weeks, there have been sustained attacks on Herat, Kandahar and Lashkar Gar, stretching Afghan special forces thin and killing dozens of civilians.